A laboratory study was made to develop a simple and economic model of producing soymilk with no beany off-flavor from water soaked and dehulled soybeans by pre-heating prior to extraction.
In the first part, Korean soybean varieties suitable as raw material for soymilk production was determined out of twelve varieties that are of economic importance in Korea. The protein and fat contents of each variety and their extract-ability in soymilk were of prime consideration. Also considered were the sensory characteristics of soymilk prepared from each variety in terms of its color, emulsion stability and flavor change during 5 months of storage at $5^\circ$C. Thus Heuk-tae( 흑태 ) and Chung-tae ( 청태 ) were determined to be unsuitable.
In the second part, the water soaked soybeans of Baek-tea ( 백태 ) variety were manually dehulled and dehulled soybeans heated in boiling water bath at intervals of 0, $\frac{1}{2}$, 1, 3, 5, 10and 30 minutes and immediately cooled by immersing into cold water bath. This way heat treatment was accurately controlled. The pre-heated soybeans were than prepared into soymilks by grinding in Osterizer blender and filtering to remove soybean residues. The protein, fat and total solid contents of each soymilk were estimated in order to determine their extract-ability. At the same time, sensory evaluation was conducted to determine the minimum preheat treatment necessary for removing beany off-flavor and this was compared with residual lipoxidase activity in each soymilk samples.
The results obtained are as following:
1. Pre-heating of water soaked and dehulled soybeans brought about accompanying decrease in extractability of protein and total solid contents in the soymilks; for protein extractability decrease was 18.21, 21.30, 45.99, 53.70, 56.17, and 63.35% after $\frac{1}{2}$, 1,3,5,10 and 30 minutes of pre-heating and for the total solid contents, 20.87, 26.32, 41.89, 42.86, 44.14 and 53.77% after the some pre-heating intervals. The total solid contents were higher than the protein at each interval, however, their extractability decreased almost in parallel.
2. 53.70% of total protein extractability decrease occurred during the first 5 minutes of pre-heating and no further significant decrease was noted even after 30 minutes of pre-heating. The soymilk prepared from water soaked and dehulled soybeans pre-heated for 5 minutes was completely free from beany off flavor. The residual lipoxidase activity in the soymilk was 17% of initial level present in soymilk prepared from unheated soybeans, corresponding to 83% of destruction.
3. Sensory score of soymilk samples improved with preheat treatment, however, with pre-heating of the water soaked and dehulled soybeans for more than 3 minutes accompanied with increasing degree of undesirable cooked flavor also. Thus the soymilk prepared from the soybeans pre-heated for 3 minutes received, the highest overall sensory rating.
4. By heating the soymilks having a slight degree of heavy off-flavor in boiling water for 5 minutes, their sensory rating could be significantly improved. However, the sensory rating of soymilks prepared from the soybeans with no and $\frac{1}{2}$ minutes of preheat treatment could not be improved by means of the 5 minutes of heating, indicating that an excessive beany off flavor once formed in soymilk could not be removed easily by subsequent heating.
5. In order to arrive at an economic processing method, formation of beany off flavor in soymilk could be mostly prevented by means of 1 minute of pre-heating the soybeans prior to extraction process and the slight amount of beany off-flavor formed in soymilk could be removed below detectable level by 5 minutes of subsequent heating. This combined process would enable to produce soymilk free from beany off-flavor and cooked flavor with a minimum sacrifice in extractability.
6. Further studies shall be required to produce more data necessary to translate the results of the study into commercial application.
경제적 및 외관상으로 중요하다고 인정되는 한국산 대두 12종류를 선정하여 이들 품종으로 soymilk 제품을 만들었을 때의 일반분석 및 품질면에서의 성질을 조사하였고, 또, soymilk 제품을 만들때 생기는 콩 비린내의 발생을 방지하기 위해 침지한 콩을 열처리하였을 때의 효과 및 이때 생기는 현상들을 분석하였다. 열처리하는 방법의 기준의 균일화를 통하여 통일된 결과를 얻기위해 침지하여 껍질을 벗긴 콩을 끊는 물속에 넣어 시간을 달리하여 처리하는 열량의 차이를 주었으며, 앞으로 대규모 처리방법으로 개발하기위한 기본 자료를 구했고 다음과 같은 결과를 얻었다.
1. 한국산 대두의 soymilk 제조 적성에서 흑태, 청태, 힐 및 유태가 soymilk 적성에서 열등한 것으로 판단되었다.
2. 열 처리에 의해 soymilk 의 총 고형물은 1분까지 급격히 감소하였고 이후 완만한 감소를 나타내었다. 1분의 가열에 의해 26.32%의 총고형물량의 감소가 있었다.
3. 단백질은 가열시작후 5분 경과할때까지 심한 감소가 있었으며 1분간의 가열로 인해 21.30%의 단백질양의 감소가 있었다.
4. 지방함량은 열처리에 의해 비교적 감소하는 정도가 적었으며, 10분간 가열하였을때 부터 지방함량의 서서한 감소가 있었으나 그 폭은 크지 않았다.
5. 제품의 emnlsion stability는 가열시작 3분이 될때까지는 점점 좋아졌고 이후 비슷한 값을 나타내었다.
6. 3분간의 열처리로 인해 콩 비린내의 발생이 나타나지 않는것으로 관능검사 결과에 의해 판단되었으며 가열시간이 10분이지나면 누린내(cooked flavor)가 발생하기 시작하는 것이나타났다.
7. Lipoxidase 는 가열시작후 1분될때까지 그 파괴되는 정도가 극심하였으며 처음 존재량의 약 83%에 해당하는 양이 3분의가열로 파괴되며 이 정도의 파괴가 콩 비린내 방생을 억제하는것으로 나타났다.
8. 이미 생성된 off-flavor 제거를 위해 제품을 후열처리 하였을때 냄새의 발생이 그리 심하지 않을때는 다소 효과가 있는것으로 조사되었다.
